Development of investigational agents targeting ion channels of cells has been increasing its importance in development of new drugs. The conventional assays using living cells have been conducted using measurement of membrane potential with a patch-clamp electrode, optical measurement of membrane potential using a voltage sensitive dye, and so on. For example, Non Patent Literature 1 discloses the technology to dye a cell with a fluorescent pigment and obtain a distance between the cell mounted on a substrate, and the substrate (degree of adhesion of the cell), using a fluorescence interference contrast microscope image, and the technology to dye a cell with a voltage sensitive dye and optically obtain a change of membrane potential of the cell. Furthermore, Non Patent Literature 2 discloses the technology to dye (or color) a cell with a FRET dye and calculate a change of cell membrane potential using the FRET phenomenon.